I know there were lots of different WILLIAMS families at Kapunda.
And I also know records do have mistakes with names.
It appears the births of the WILLIAMS (nee CARTER) children had their father always recorded with William as his first name and none were born at Kapunda. But John was only the name recorded with the DUNKLEY connection. And I think it is quite odd.
A John WILLIAMS aged 56y died at Kapunda 28 Jun 1899 and he was a similar age to "John" who married Mary Ann DUNKLEY.